Another different idea that just came to me relates to the "pendulum fallacy", whereby people mistakenly think that a rocket is hanging by the nose from an imaginary string, instead of actually resting on top of its tail/thruster.

So then, what if you could position a lot more of the lander's mass below the thruster?Then you really would have a little more basis for pendulum paradigm.And a pendulum is a little more easy to balance on a vertical centerline than a pencil standing on its end is.

So what if your rocket/lander looked more like a bell shape, where the thruster was located on the inside of the bell near the top, and the fuel tanks with their propellant mass were located on the outside of the bell near the bottom?Wouldn't that type of mass distribution make it easier to do your balancing act, since your vehicle's mass distribution would be helping you rather than working against you?

Comments? Thoughts?

The Pendulum fallacy is the idea that if you put the entire rocket beneath the engine it would behave any differently than if the engine was at the bottom. Goddards Rocket had the fuel tanks beneath the rocket in the center of the plume. It didn't help.

Or could you just have the lone main rocket thruster fixed in position, and then have some additional smaller thrusters at the very top of the craft to tip the nose in the right direction to maintain balance?

Attitude thrusters. And yes, we've done that too (in the really early days, and the thrusters were small peroxide engines, outboard instead of at the top):

Another different idea that just came to me relates to the "pendulum fallacy", whereby people mistakenly think that a rocket is hanging by the nose from an imaginary string, instead of actually resting on top of its tail/thruster.

So then, what if you could position a lot more of the lander's mass below the thruster?Then you really would have a little more basis for pendulum paradigm.And a pendulum is a little more easy to balance on a vertical centerline than a pencil standing on its end is.

So what if your rocket/lander looked more like a bell shape, where the thruster was located on the inside of the bell near the top, and the fuel tanks with their propellant mass were located on the outside of the bell near the bottom?Wouldn't that type of mass distribution make it easier to do your balancing act, since your vehicle's mass distribution would be helping you rather than working against you?

I see that the ARCA team is planning to launch soon with just such a pendulum rocket! First a balloon and then rocket engines all tied together like a line of hotdogs!

What has that to do with the pendulum rocket fallacy? Yes, their rocket IS hanging from the balloon like a pendulum to gain altitude, but it has nothing to do with a "self-stabilizing" rocket, it just launches from a balloon .

The later stages of the ARCA rocket use non-rigid mechanics (cables) between the stages.

Quote:

Helen will be using a gravitational stability method in vertical flight without aerodynamic surfaces or jet commands, by towing the component stages and payload. The towing can be made by cable, or rigid-articulated system.In order to have a stabilized rocket in vertical flight, in gravitational field, this method is using a towed mass in the same direction with the thrust. This mass consists in the next rocket stages and payload. The stability effect depends of several elements: the mass of the stabilized body, the mass of the towed mass and the length between the centers of gravity of the stabilized body and the stabilizer body.This method can be applied in the extra atmospheric space on vertical ascendant or descendent trajectory under the influence of gravitational field.

Though I don't have any way of proving this, I believe that this really is just another case of the pendulum rocket fallacy. If this does anything to stabilize the rocket, then it does it aerodynamically. I would really like to know the reasoning behind their statement, that this works with gravity and can be used outside the atmosphere...

John, for all the achievements of AA, a decade has passed and you are still bouncing on the ground, no disrespect intended. But at this pace and with the commitment to bouncing turists (which I assume will be very demanding on AA time) when will you go for orbit?have you done the math and the design for a low weight payload vehicle to orbit?do you realize that a space telescope (both for looking up and down) can be made very light and even a 50cm mirror can resolve 20cm resolution from 250km up? is there not commercial application to that? mapping companies and universities buying telescope time? indeed someone like google might well want to pay for everything just for the mapping capability or the chance of it.

I believe it is/was your stated motivation of AA that Nasa is far too ineffective and slow, yet a decade has passed and it looks like some measure of subconscious capitulation has taken place, being distracted by competitions, small contracts and now bouncing turists. When will you go for orbit? Can you promise that another decade will not pass without at least seriously trying for orbit?

This might be hard to hear when used to all the praise of people here but I feel very strongly that you are missing a great opportunity to do something that really will make a difference because unlike most, AA is a combination of your skill and your financial means and orbit is where the difference is made, that's where Nasa will be put to shame and it is so needed.

I would ask that you at least do a design and do the math projections on how it would be and what it would cost so you will at least have seriously considered the possibility. So you are armed with the awareness of the possibility.I would ask that you do the math on the diffraction limit of a 50cm mirror and compare the values to the hubble telescope pixel size. I think you will find it interesting. hubble doesn't even have color and only a 1024x1024 imager.

ask yourself, how light can a space mirror be, how light can an imager be, how light can a radiocircuit be, how light can a battery be, the solar film, the controlling electronics, the gyro mechanics for orientation. and how light can the rocket be to lift it there. and how many universities would stand in line to manufacture the mirror for you if you can put it in space for them.

imagine seeing that live HD image of the earth rolling by and being able to switch to 20cm resolution on the ground as it rushes by. you could see yourself wave to it.

I am not a rocket scientist, but I suspect that a turbine + generator large enough to produce sufficient electrical power to an arcjet would be heavier than the gimbals you are attempting to replace. Also, I've read that the typical life of an arcjet assembly is about 1500 hours of operation. If the price of an arcjet is higher than that of the gimbals, then this solution increases the weight and cost of the rocket. I'm not sure that it necessarily increases the reliability of the rocket.

Hi,

Thanks for that response. Yeah, I was wondering about that too, after I posted. So what is the mechanism used for gimbaling? Is it some sort of electrical/magnetic solenoid type of actuator?

Quote:

Helium is a nice gas to use in a cryogenic system. It is lightweight, won't liquify or freeze against LOX and it is non-flammable.

Well, they say that Helium is a precious and dwindling substance, so I certainly hope there's enough of it left to supply frequent lunar travel in the later part of this century.

Meanwhile, I feel that energy is a more fungible and abundant commodity in space, which also happens to have a lot of raw energy coursing through it, so that a space vehicle which is extravagant on energy while being conservative on propellant might be the best compromise. Electric thrusters seem to lean in that direction.

Thanks for that response. Yeah, I was wondering about that too, after I posted. So what is the mechanism used for gimbaling? Is it some sort of electrical/magnetic solenoid type of actuator?

That's something that team Armadillo needs to answer. I don't know what they are using. I suspect it would be something like a solenoid.

sanman wrote:

Well, they say that Helium is a precious and dwindling substance, so I certainly hope there's enough of it left to supply frequent lunar travel in the later part of this century.

I've read some people speaking with alarm at the casual way we use helium these days (filling party balloons with it seems like a waste to these people). I don't know what to think of their warnings. Helium is produced naturally by radioactive decay.

As for space flight, it is my understanding that pumps reduce or eliminate the need for gas pressurization.

An aerospike engine is only useful across a wider range of ambient pressure, whereas with a lunar lander you've got no atmosphere at all, and even for the purpose of this Level2 test, you weren't going upto a very high altitude.

So aerospike would be good if you're going from ground to orbit, and thus going across the full range of ambient pressure.